alerts: Consolidate all the low disk free alerts into a single rule.
alerts: Consolidate all the low disk free alerts into a single rule.

BUG=skia:

Review URL: https://codereview.chromium.org/1736003002

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot